Efficacy is the capacity to produce an effect. It has different specific meanings in different fields. In medicine, it is the ability of an intervention or drug to reproduce a desired effect in expert hands and under ideal circumstances.
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In a healthcare context, efficacy indicates the capacity for beneficial change (or therapeutic effect) of a given intervention (e.g. a medicine, medical device, surgical procedure, or a public health intervention). If efficacy is established, an intervention is likely to be at least as good as other available interventions, to which it will have been compared. Comparisons of this type are typically made in 'explanatory' randomized controlled trials, whereas 'pragmatic' trials are used to establish the effectiveness of an intervention.
The concept of 'self-efficacy' is an important one in the self-management of chronic diseases because doctors and patients often do not follow best practice in using a treatment. For instance, a patient using combined oral contraceptive pills to prevent pregnancy may sometimes forget to take a pill at the prescribed time; thus, while the perfect-use failure rate for this form of contraception in the first year of use is just 0.3%, the typical-use failure rate is 8%.[1]
Self-efficacy is a proximal and direct predictor of intention and of behavior. According to Social Cognitive Theory, a personal sense of control facilitates a change of health behavior.[1] Self-efficacy pertains to a sense of control over one's environment and behavior. Self-efficacy beliefs are cognitions that determine whether health behavior change will be initiated, how much effort will be expended, and how long it will be sustained in the face of obstacles and failures. Self-efficacy influences the effort one puts forth to change risk behavior and the persistence to continue striving despite barriers and setbacks that may undermine motivation. Self-efficacy is directly related to health behavior, but it also affects health behaviors indirectly through its impact on goals. Self-efficacy influences the challenges that people take on as well as how high they set their goals (e.g., "I intend to reduce my smoking," or "I intend to quit smoking altogether"). Individuals with strong self-efficacy select more challenging goals.[2] They focus on opportunities, not on obstacles (e.g., "At my university there is a smoking ban, anyway," instead of "There are still a lot of ashtrays at my university").
In an insolvency context, in particular in relation to an Individual Voluntary Arrangement, the efficacy of such an arrangement is judged by the extent to which it is achieving its aims of getting money in to pay to the creditors of the debtor.
In pharmacology, efficacy, refers to the maximum response achievable from a drug. It is often described by the parameter Emax.[3] Intrinsic activity is a relative term which describes the efficacy relative to a drug which has the highest observed efficacy. Effectiveness refers to the ability of a drug to produce a beneficial effect. A distinction is made between 'method' effectiveness which describes the effect achievable if the drug was taken as prescribed and 'use' effectiveness which is the effect obtained under typical use circumstances when adherence is not 100%. The widely used 'intention to treat' method of analysing clinical trials provides estimates of 'use' effectiveness which are typically biased compared with 'method' effectiveness.
In lighting design, "efficacy" refers to the amount of light (luminous flux) produced by a lamp (a light bulb or other light source), usually measured in lumens, as a ratio of the amount of power consumed to produce it, usually measured in watts. This is not to be confused with efficiency which is always a dimensionless ratio of output divided by input which for lighting relates to the watts of visible power as a fraction of the power consumed in watts. The visible power can be approximated by the area under the Planck curve between 300 nm and 700 nm for a blackbody at the temperature of the filament as a ratio of the total power under the blackbody curve. Efficiency values for light from a heat source are typically less than two percent.
In Lutheranism efficacy is used to describe the characteristics of Scripture. The efficacy of Scripture means that it is united with the power of the Holy Spirit and with it, not only demands, but also creates the acceptance of its teaching[4][5][6] and that this teaching produces faith and obedience. Efficacy further means that Holy Scripture is not a dead letter, but rather, the power of the Holy Spirit is inherent in it[7][8][9][10][11] and that Scripture does not compel a mere intellectual assent to its doctrine, resting on logical argumentation, but rather it creates the living agreement of faith.[12][13][14][15] The Smalcald Articles affirm, "in those things which concern the spoken, outward Word, we must firmly hold that God grants His Spirit or grace to no one, except through or with the preceding outward Word."[16] The Formula of Concord teaches that when humans reject the calling of the Holy Spirit, it is not a result of the Word being less efficacious. Instead, contempt for the means of grace is the result of "the perverse will of man, which rejects or perverts the means and instrument of the Holy Ghost, which God offers him through the call, and resists the Holy Ghost, who wishes to be efficacious, and works through the Word..."[17]
The efficacy of a differential amplifier is measured by the degree of its rejection of common-mode signals in preference to differential signals. Referred to as common-mode rejection ratio (CMRR); typically specified in decibels.